1.Technical Field
The present invention relates to nickel based alloys having high hardness and compressive yield strength. Such alloys are especially useful for engine parts such as valve seat inserts. Another aspect of the invention relates to part s made of such alloys.
2. Related Prior Art
Nickel based valve seat insert alloys generally have wear resistance, heat resistance, and corrosion resistance superior to those f high alloy steels, and are therefore often used as materials for structural members serving under severe conditions, such as valve seat inserts. Known nickel based alloys used for exhaust valve seat inserts, such as an alloy identified as J96 and marketed by L. E. ones Company, have relatively good characteristics, including good hardness and compressive yield strengths.
Valve seat inserts made of such known alloys may experience wear in some heavy duty engine applications. Such wear may be caused, in part, by a lack of fineness and uniformity in the microstructure of the alloy. Microstructure refers to carbide size and distribution within a matrix material; a finer microstructure has a greater number of smaller car ides distributed throughout the matrix material. Such wear may also be caused, in part, by a weak matrix material which does not adequately support the carbides. Therefore, a nickel based alloy having more fineness and uniformity in the microstructure and a high matrix strength resulting in higher hardness and compressive yield strength is desirable.
The present invention is a nickel based alloy which has good fineness and uniformity in the microstructure in combination 4 with a high matrix strength resulting in increased hardness and compressive yield strength properties. These properties provide an alloy having increased wear resistance at elevated temperatures. Also, when the alloy is used as a material or a valve seat insert, the increased compressive yield strength of the alloy provides an insert having improved retention, i.e., the valve seat remains properly seated in the cylinder head counterbore.
Cobalt-base materials, such as alloy identified as J3 and marketed by L. E. Jones Company, which tend to demonstrate good wear properties, have also been used to manufacture valve seat inserts. Utilizing the alloy of the present invention, goo wear properties may be obtained without the addition of relatively expensive cobalt material.
In one aspect, the preset invention is an alloy which comprises:
In another aspect, the present invention is a nickel based alloy which contains, in weight percent;
chromium from about 34.0 to about 37.0;
iron from about 5.0 to about 7.0;
molybdenum from about 2 to about 6.25; and
the balance nickel, other alloying elements, and incidental impurities;
wherein the chromium, iron, and molybdenum content of the alloy increase the fineness and uniformity of the microstructure and increase the matrix strength of the alloy thereby resulting in the alloy having increased hardness and compressive yield strength.
In another aspect, the present invention is a cast alloy comprising chromium, nickel, tungsten, iron and molybdenum, wherein the relative concentration of Cr, Ni and W is such that a three-phase eutectic composition is capable of forming at a temperature of about 800xc2x0 C.
The present invention is a substantial modification over existing industry standard nickel based alloys. One of the modifications over existing industry standard nickel based alloys may be accomplished by increasing the chromium and adding molybdenum to the alloy. The microstructure of the resulting alloy is much finer and more uniform than that of existing standard alloys. The molybdenum serves to increase the matrix and grain boundary strength.
In another aspect of the invention, metal parts such as valve seat inserts are made from the alloy.
In addition to higher hardness and higher compressive yield strength, the preferred alloys of the present invention also tend to have good wear resistance, good corrosion resistance and good oxidation resistance.
Before embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of the composition and concentrations of components set forth in the following description. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.
The present invention is directed to a nickel based alloy with improved hardness, compressive yield strength, and wear resistance. The alloy is designed particularly for use in internal combustion engine valve seat inserts, but many other applications are feasible. The present invention is based on experimental findings that hardness and compressive yield strength of the nickel based alloys can be significantly increased by increasing the matrix strength and improving the fineness and uniformity of the resulting microstructure; this may be achieved by increasing the chromium content to higher levels, and by adding molybdenum to the nickel based alloy.
Carbon (C) is present in the alloy in an amount ranging from about 2 to about 3 weight percent of the total alloy; preferably, at least about 2.2 weight percent; preferably, at most about 2.6 weight percent.
Chromium (Cr) is present in the alloy in an amount ranging from about 30 to about 40 weight percent of the total alloy. The chromium content should be high enough to achieve a three-phase eutectic microstructure with nickel and tungsten at a temperature of about 800xc2x0 C. Preferably, the chronium content is at least about 34.0 weight percent. Preferably, the chromium content is at most about 37.0 weight percent.
Tungsten (W) is present in the alloy in an amount ranging from about 12 to about 18 weight percent of the total alloy. Preferably, the tungsten content is at least about 14.0 weight percent. Preferably, the tungsten content is at most about 16.0 weight percent.
Iron (Fe) is present in the alloy in an amount ranging from about 3.5 to about 8.5 weight percent of the total alloy; preferably, at least about 5.0 weight percent. Preferably, the iron content is at most about 7.0 weight percent. Preferably, the Mo and Fe content is controlled so the combination of iron and molybdenum tends to form an intermetallic Laves phase Fe2Mo which strengthens the grain boundaries. The temperature and relative concentration of Mo and Fe to form a Laves phase may be determined by reference to a Fexe2x80x94Mo binary phase diagram. Such a diagram is shown, for example, on page 2-197 of the ASM Handbook, Copyright 1992, Volume 3, which is herein fully incorporated by reference.
Molybdenum (Mo) is present in the alloy in an amount ranging from about 1 to about 8 weight percent of the total alloy. Generally, greater molybdenum increases alloy hardness and decreases carbide size; however, too much molybdenum may result in a brittle product. The weight percent molybdenum is preferably at least about 2 weight percent. The weight percent molybdenum is preferably at most;about 6.25 weight percent. Most preferably, the alloy contains about 4 to 5 weight percent Mo.
Manganese (Mn) can be added or present in an amount of up to about 0.5 weight percent of the total alloy.
Silicon (Si) may be added to or present in the alloy at levels up to about 1.0 weight percent of the total alloy.
The balance of the alloy is nickel (Ni) and incidental impurities. Generally, the alloy contains at least about 20 weight percent nickel; preferably at least about 30 weight percent.
At 800xc2x0 C., the matrix material between the carbides preferably contains a three-phase eutectic composition of the elements Crxe2x80x94Nixe2x80x94W, which provides increased strength. The relative concentration of Crxe2x80x94Nixe2x80x94W necessary to form a three-phase eutectic composition may be determined by reference to a Crxe2x80x94Nixe2x80x94W ternary component phase diagram. Such phase diagrams are shown, for example, on page 3-48 of the ASM Handbook, Copyright 1992, Volume 3, which is herein incorporated by reference.
In a highly preferred embodiment, the alloy comprises:
Metal parts are either made from the alloy, such as by casting or forming from a powder and sintering, or the alloy is used to hardface the parts. Preferably, the alloy is manufactured by casting. Casting is a conventional process in which raw materials are added together and melted to liquid state, and then poured into a cast mold.
Preferably, the metal parts are valve seat inserts for use in internal combustion engines.
Data obtained on an alloy embodying the alloy of the invention is shown in the tables listed in Example 1.
Although the alloy is nickel-based, the thermal expansion coefficient of the alloy tends to be closer to that of iron than nickel. (The thermal expansion coefficient of cast iron is approximately 11.5xc3x9710xe2x88x926 mm/mm xc2x0 C. at a temperature of 25-600xc2x0 C.) This is beneficial because the insert tends to be much hotter than the surrounding material when the engine is operating. If the thermal expansion coefficient of the insert alloy closely matches that of the cylinder head alloy, this enables the insert and cylinder head to expand at the same rate, thereby improving insert retention characteristics.
The alloy has good high temperature compressive yield strength which increases wear resistance and decreases material yielding during operation. Decreased yielding serves to improve insert retention. Preferably, the alloy has a compressive yield strength of at least about 110 thousand pounds per square inch (KSI) at room temperature; more preferably, about 130 KSI at room temperature.
Increased hot hardness contributes to improved wear resistance and provides a safety factor for inserts which run beyond the normal operating temperature.